The present invention relates to a quantum state control apparatus, an optical receiver and an optical communication system which can greatly improve the receiving sensitivity and bit error rate in coherent optical communication
The signal-to-noise ratio in a coherent optical communication system is limited by the quantum noise contained in a signal lightwave, the noise caused by a local oscillator, and a dark current and thermal noise in the optical receiver At present, the above signal-to-noise ratio can be increased to a level determined by the quantum noise contained in a signal lightwave. FIG. 2 shows an example of a conventional coherent optical communication system. In FIG. 2, reference numeral 1 designates an optical transmitter for sending out a signal lightwave with a coherent state, 2 a transmission line for transmitting the signal lightwave and 3 an optical homodyne detector. The optical homodyne detector is explained in Chapter 6 (in more detail, on pages 228 and 229) of a book entitled "Optical Communication Theory and Its Application", and published by Morikita Publishing Co. in 1988 (the above book will be hereinafter referred to as "reference 1"). Recently, in order to further improve the above signal-to-noise ratio, it has been proposed to utilize a physical phenomenon capable of controlling the quantum state of light and reducing the quantum noise contained in a signal lightwave. That is, a technique for generating a squeezed state (or two-photon coherent state) is used (Physical Review, A13, No. 6, June, 1976, pages 2226 to 2243). The term "squeezed state" means a state in which two quadrature components (namely, two complex amplitude components perpendicular to each other) of light are different in quantum noise from each other. On the other hand, the light used in a conventional coherent optical communication is the so-called coherent state. In other words, the two quadrature components of this conventional coherent light are equal in quantum noise to each other.
It is now proposed to use the above quantum state control technique (that is, squeezed-state generating technique) for the purpose of controlling the quantum state of a signal lightwave in an optical transmitter, as shown in FIG. 3. In FIG. 3, reference numeral 4 designates an optical transmitter for sending out a signal lightwave with a squeezed state, and reference numerals 2 and 3 designate the same parts as shown in FIG. 2. When a signal lightwave has a coherent state, the quantum noise of each of two quadrature components of the signal lightwave is 1/4 (in normalized power). However, when the signal lightwave has a squeezed state, the quantum noise of one of the two quadrature components can be made equal to 1/4e.sup.-Z, and the quantum noise of the other component can be made equal to 1/4e.sup.Z (where Z indicates a squeezed parameter). As a result, where the quantum state of transmitted light is the coherent state as in ordinary coherent optical communication, the theoretical value of the signal-to-noise ratio of the communication system is equal to 4&lt;n&gt;. On the other hand, where the transmitted light has a squeezed state, and one quadrature component low in quantum noise is used as an information signal, the theoretical value of the signal-to-noise ratio is equal to 4&lt;n&gt;(&lt;n&gt;+1). The above symbol &lt;n&gt; indicates the mean value of the number of photons included in a single optical signal pulse. As is evident from the above, the signal-to-noise ratio of a coherent optical communication system can be improved in a great degree by putting signal lightwave in a squeezed state. Details of such a matter are described in Chapters 1 and 2 (in more detail, on pages 34 to 36 and 48 to 51) of the reference 1.
However, in a case where the quantum state of signal lightwave is controlled in the optical transmitter as mentioned above, the squeezed state of the signal lightwave is destroyed when encountered in the transmission line for communication. Thus, the signal-to-noise ratio of the communication system is decreased, and the advantage of the quantum state control technique is lost. That is, when the quantum state control technique is applied to an actual communication system, there arises a series problem. Details of the above phenomenon are described in Chapter 2 (in more detail, pages 48 to 51 and FIG. 2.8) of the reference 1.
As mentioned above, the conventional quantum state control technique for improving the transmission characteristics in coherent optical communication cannot produce an expected effect when an energy loss is encountered in a transmission line for communication.